So I've got one of these Takumi cup bases and an H&S CR+ micro-cup in hand now, and I want to post some clarification to what DaveG said:
- The H&S cup doesn't fit perfectly. The threads match, but the threaded section on the H&S cup is shorter than the threaded section in the Iwata base, so there's a gap of about 1mm inside the cup, instead of the parts meeting flush. This is why Dave has that black O-ring in there: you need something like that to fill that gap.
- Neither the white teflon H&S cup O-rings, nor the red rubber Neo cup O-rings will fit in there. Both are too large in diameter for the bottom of the Takimu cup. The Neo can be crammed in there since the material is compressible and the ring cross section is thinner, but its cross section is thinner than the gap, so it engages by rippling rather than actually sealing. The H&S ones cannot fit at all without folding/pinching a significant bit of section into the cup, so they're a total no-go. You will need to source different O-rings in order to use H&S cups with the Iwata base.
*Edit- The silicone keyboard switch O-rings I've successfully used elsewhere on my brushes don't work either: they're too small in diameter.